This invention relates to permanent magnets, and particularly to termination structures for permanent magnets which do not distort the magnetic field.
A permanent magnet designed for applications such as medical clinical use is an open structure with opening dimensions dictated by the size of a human body. An open magnetic structure makes it impossible to achieve a perfectly uniform magnetic field within the region of clinical interest. Thus a major problem in magnet design is the partial compensation of the field distortion generated by the magnet opening in order to achieve the degree of uniformity dictated by the diagnostic requirements within the region of interest.
An important category of permanent magnet is a structure of permanent magnetized material designed to generate a uniform magnetic field within the cavity of the magnet and to contain the field within the volume of the magnet without the use of external magnetic yokes or magnetic shields. Materials like ferrites and high energy product rare earth alloys are suitable for this category of permanent magnets.
The two conditions of field uniformity and field confinement can be attained in cylindrical structures where the magnetic configuration consists of a series of concentric layers of magnetized material. In practice the cylindrical structure has to be truncated and the effect of the opening becomes less and less important as the length of the cylinder becomes larger and larger compared to the cylinder transversal dimensions. From a practical standpoint, the optimum design of the termination is the one that minimizes length and weight of the magnet.
It is accordingly the principal object of the invention to optimize the termination of a cylindrical permanent magnet structure with a minimum distortion of the field inside the magnet cavity and a minimum field leakage outside of the magnet.
It is a further object of this invention to provide a perfect termination for a closed magnet.
It is another object of the invention to provide a magnetic structure termination for the partial closing of a structure of multiple concentric layers.